In insects, the olfactory pathway starts at the antennae (though in some insects like Drosophila there are olfactory sensory neurons in other parts of the body) from where the sensory neurons carry the information about the odorant molecules impinging on the antenna to the antennal lobe. The antennal lobe is composed of densely packed neuropils, termed glomeruli, where the sensory neurons synapse with the two other kinds of neurons, the projection neurons and the local neurons. There are 43 glomeruli in the Drosophila antennal lobe. The projection neurons project to higher brain centers such as the mushroom body and lateral horn of the protocerebrum. The local neurons, which are primarily inhibitory, have their neurites restricted to the antennal lobe. In Drosophila, each olfactory sensory neuron generally expresses a single olfactory receptor gene, and the neurons expressing a given gene all transmit information to one or two spatially invariant glomeruli in the antennal lobe. Moreover, each projection neuron generally receives information from a single glomerulus. The interaction between the olfactory receptor neurons, local neurons and projection neurons reformats the information input from the sensory neurons into a spatio temporal code before it is sent to higher brain centers.

The plasticity and coding properties of the projection neurons are currently under intensive investigation at several laboratories.